In the growing of semiconductor crystals from a "melt" of molten semiconductor it has sometimes become necessary to transfer the semiconductor at a high temperature on the order of 1425.degree. C. from one vessel to another without encountering solidification or contamination by contact with impurities.
For example, in the production of polycrystalline silicon it may be necessary to melt various sized pieces of the silicon raw material in a central crucible to produce a reservoir or "melt", over which a slag consisting principally of silicon oxide forms. The silicon is then transferred from a point within the central melt by means of various conduits or transfer tubes to remote locations for utilization in casting or crystal-growing operations. Several advantages accrue from this arrangement. By extracting the molten silicon from a point well within the melt and away from the slag on the surface, a considerable elimination of silicon oxides and other impurities is achieved. Further, the removal to a separate location of the growing or casting operations leaves the central meltdown crucible available for the addition of fresh supplies of silicon raw material.
Similarly, in the growing of high quality monocrystalline boules by the Czochralski process it has recently become possible to significantly reduce the cost of the boules and, consequently, the cost of the wafers sliced therefrom, by resort to a continuous production process in which silicon in polycrystalline form is melted in a meltdown crucible at the same time that the monocrystalline boules are being drawn in an adjacent drawing crucible connected to the meltdown crucible by a conduit. Such a system is described and claimed in our commonly assigned copending patent application Ser. No. 83,169 filed concurrently herewith and granted as U.S. Pat. No. 4,282,184 on Aug. 4, 1981.
Unfortunately, the task of transferring molten high purity silicon from one crucible to another without any significant loss in quality enroute is attended with a number of difficult problems.
Since the silicon must be maintained in its initial high purity state, it may not be permitted to come in contact with a wide range of materials while it is molten and at a high temperature because of its ready ability to form alloys and compounds with these materials. In practice, the only satisfactory material from which to fabricate both the crucibles and the transfer tube is quartz which is, chemically, silicon dioxide (SiO.sub.2).
At the temperature of molten silicon, even quartz is fairly rapidly reduced and eroded away. Fortunately the rate of erosion is tolerable although expensive. More importantly, the byproducts, silicon and oxygen, are fairly compatible with the melt being transferred. Nevertheless, since erosion is significant, replacement of crucibles and transfer tubes must be made as easy and simple as possible.
Another significant fault of quartz when used as a means of storing or transporting molten semiconductor materials is its relative mechanical weakness at the temperatures encountered. The softening temperature of pure quartz is approximately 1600.degree. C. However, the quartz actually available at a reasonable cost contains some impurities which cause significant softening and weakening even at the approximately 1425.degree. C. temperatures encountered in use with molten silicon.
Other problems also beset the designer of a conduit for transferring molten silicon. In practice, the silicon can rarely be raised far above its fusion temperature without significant increases in erosion and softening of the quartz. In fact, the melt must be maintained substantially at the fusion temperature in a crucible from which a boule is being grown by the Czochralski process. Consequently, there is always a significant danger of unwanted solidification occurring in the transfer tube and causing an abrupt and expensive termination of operations.
Thorough insulation of the transfer tube has not proven by itself to be an adequate solution to the problem of solidification. Although heat loss can be significantly reduced by this means, the silicon is operating at a temperature so close to its fusion point that even relatively minor heat losses can precipitate a disastrous solidification and blockage of a transfer tube. Consequently, it has become necessary to incorporate a heater extending in some fashion along the transfer tube to supply heat lost in transit of the molten silicon from the meltdown crucible to a remote location.
Even the use of such a heater overlaying the transfer tube has not proven entirely satisfactory in eliminating unwanted solidification within the tube. The problem of solidification continues to exist even though the power input to the transfer tube is adequate, in combination with the thermal insulation provided, to hold the temperature above the fusion point of the molten silicon. Careful investigation of the temperature along the length of the quartz transfer tube, however, has revealed the cause of these continuing solidifications.
The type of conduit under consideration is one in which the quartz transfer tube, the innermost member which actually carries the molten silicon, extends downwardly through the surface slag layers of a meltdown crucible, and may also extend downwardly into a utilization crucible at its other end, remote from the meltdown crucible.
In order that the outer thermal insulation layers of the conduit not contact the molten silicon, it is necessary to terminate the thermal insulation layer surrounding the transfer tube at a point short of contact with the molten semiconductor. Given reasonable manufacturing tolerances and the necessity to accommodate some change in melt level, this has meant in practice that a certain length of the quartz tube between the end of the thermal insulation layer and the surface of a melt had to be left exposed. By thermocouple measurement of the temperature profile along the transfer tube, the heat losses from these exposed portions were shown to be unexpectedly large and to constitute a significant problem. Consequently, even after the incorporation of heaters extending fully along the length of the insulated portion of the transfer tube there have been occasions of recurrence of the solidification problem.